Numerous geodetic surveying appliances have been known for surveying a target since ancient times. In this context, the direction and the angle and usually also the distance from a measuring appliance to the target that is to be surveyed are recorded and also, in particular, the absolute position of the measuring appliance together with any reference points present are captured as spatial standard data.
Generally known examples of such geodetic surveying appliances are a theodolite, a tacheometer and a total station, which is also called an electronic tacheometer or computer-tacheometer. A geodetic measuring apparatus from the prior art is described in the publication document EP 1 686 350, for example. Such appliances have electrosensory angle and possibly distance measuring functions which allow a direction and a distance to a selected target to be determined. In this case, the angle and distance variables are ascertained in the internal reference system of the appliance and may also need to be linked to an external reference system for absolute position finding.
In many geodetic applications, points are surveyed by placing specially designed target objects at said points. These usually consist of a plumb rod having a reflector (e.g. a 360-degree prism) for defining the measurement path or the measurement point. For such surveying tasks, the measurement process is controlled and measurement parameters are stipulated or registered by transmitting a number of data items, instructions, speech and other information between the target object—particularly a hand-held data capture appliance on the part of the target object—and a central measuring appliance. Examples of such data are the identification of the target object (type of prism used), the inclination of the plumb rod, the height of the reflector above ground, reflector constants or measured values, such as temperature or air pressure. This information or these situation-dependent parameters is/are necessary in order to allow high-precision sighting and surveying of the measurement point defined by the plumb rod with the prism.
Modern total stations have microprocessors for digital further processing and storage of captured measurement data. The appliances usually have a compact and integrated design, with usually coaxial distance measuring elements and also computation, control and memory units being present in one appliance. Depending on the expansion level of the total station, there may also be integrated motorization of the sighting and targeting device and—if retroreflectors (for example a 360-degree prism) are used as target objects—means for automatic target searching and tracking. As a man-machine interface, the total station may have an electronic display control unit—generally a microprocessor computation unit with electronic data storage means—having display and input means, e.g. a keypad. The display control unit is supplied with the measurement data captured by electrosensory means, with the result that the position of the target can be ascertained, visually displayed and stored by the display control unit. Total stations known from the prior art may also have a radio data interface for setting up a radio link to external peripheral components, such as a hand-held data capture appliance, which may be in the form of a data logger or field computer, in particular.
To sight or target the target that is to be surveyed, geodetic surveying appliances of the type in question have a telescopic sight, such as an optical telescope, as a targeting device. The telescopic site is generally able to be rotated about a vertical axis and about a horizontal tilt axis relative to a base of the measuring appliance, so that the telescope can be oriented to the point to be surveyed by means of swiveling and tilting. Modern appliances can have, in addition to the optical viewing channel, a camera, integrated into the telescopic sight and having a coaxial or parallel orientation, for example, for acquiring an image, wherein the acquired image can be presented particularly as a live image on the display of the display control unit and/or on a display of the peripheral device used for remote control—such as the data logger. The optical system of the targeting device may have a manual focus—for example an adjusting screw for altering the position of a focusing optical system—or may have an autofocus, with the focus position being altered by servomotors, for example. Such a targeting device for a geodetic surveying appliance is described in European patent application No. 09152540.2, for example. Automatic focusing devices for telescopic sights for geodetic appliances are known from DE 197 107 22, DE 199 267 06 or DE 199 495 80, for example.
The optical system or the optical viewing channel of the sighting device usually contains an objective lens group, an image reversal system, a focusing optical system, a reticle for producing cross hairs and an eyepiece, which are arranged in this order from the object side, for example. The position of the focusing lens group is set depending on the object distance such that a sharp object image arises on the reticle arranged in the focusing plane. Said image can then be viewed through the eyepiece or, by way of example, acquired using a coaxially arranged camera.
By way of example, the design of telescopic sights of the type in question for geodetic appliances is shown in the publication documents EP 1 081 459 and EP 1 662 278.
On account of the beam path that is usually to be utilized jointly both as a viewing channel and for measurements, such appliances require the technical design of said beam path in the manner of construction of a telescope with specialized, high-precision optical systems that are complex to manufacture. Furthermore, an additional separate transmission and reception channel and also an additional image plane for the wavelength of the distance measuring device are provided for the coaxial electronic distance measurement.
Since target objects (e.g. the plumb rods with a target mark such as a 360-degree prism that are usually used for geodetic purposes) can be targeted with sufficient precision with the naked eye (i.e. in accordance with non-geodetic accuracy requirements) using the sighting device, despite the 30-times optical magnification which is often provided, conventional surveying appliances in the meantime have, as standard, an automatic target tracking function for prisms used as a target reflector (ATR: “Automatic Target Recognition”). For this, a further separate ATR light source—e.g. a multimode fiber output, which emits optical radiation having a wavelength in the region of 850 nm—and a specific ATR detector (e.g. CCD area sensor) that is sensitive to this wavelength are conventionally additionally integrated in the telescope.
As part of the ATR fine targeting function, the ATR measurement beam is emitted in the direction of the optical target axis of the sighting device, said measurement beam is retroreflected at the prism, and the reflected beam is captured by the ATR sensor. Depending on the deviation in the orientation of the optical target axis from the prism, the impingement position of the reflected radiation on the ATR sensor also deviates from a central sensor area position in this case (i.e. the reflected spot of the ATR measurement beam retroreflected at the prism on the ATR area sensor is not located in the center of the ATR area sensor and therefore does not impinge at a setpoint position which has been stipulated, e.g. by means of calibration, as that position that corresponds to the optical target axis).
If this is the case, the orientation of the sighting device is slightly readjusted in motorized fashion such that the ATR measurement beam retroreflected at the prism impinges on the ATR area sensor with high precision at the center of the sensor area (i.e. the horizontal and vertical angles of the sighting device are iteratively changed and adjusted in such a way until the center of the reflected spot coincides with the setpoint position on the ATR area sensor). Alternatively, a residual deviation between the impingement point of the retroreflected ATR measurement beam on the ATR area sensor and the center of the sensor area can also be taken into account in calculations and converted into an angle which is added as appropriate to the spatial angle—captured using the angle sensors—at which the target axis points. That is to say that the spatial angle to the target could also be derived in this case from the spatial angle—captured using the angle sensors—of the target axis and a removal of the detected ATR measurement beam reflection from the sensor center (i.e. from that central point on the ATR sensor at which the target axis is depicted).
As a result, it is possible to significantly increase the attainable accuracy for the orientation of the optical target axis onto the prism in comparison with manually performed targeting with cross hairs and on the basis of mere judgement by eye. In order to ensure that the automatic targeting on the basis of evaluation of the position of the reflected spot of the ATR measurement beam retroreflected at the prism on the ART area sensors works, it is necessary—prior to starting a function—to orient the sighting device to the target reflector at least with such approximation that the retroreflected ATR measurement beam also impinges on the ATR area sensor. This can be achieved by means of, by way of example, prior manual targeting of the target reflector on the basis of judgement by eye or by means of the execution of an automatic coarse targeting function.
The manual, coarse sighting of the target object can be performed by the user firstly by viewing and targeting the target object on a user display of the display control unit directly on the surveying appliance or on the display of a separate peripheral device (e.g. a data logger as a remote control). Often, however, this continues to be accomplished by viewing the target through the eyepiece of the telescopic sight (i.e. the sighting device), since in practical use—e.g. in sunlight—a displayed display image on the display control unit or the data logger may be unsatisfactorily discernible.
Besides the ATR fine targeting function, it is also possible for an automatic target tracking functionality to be provided in similar fashion and by using the same ATR components (such as ATR light source and ATR detector). Following a performance of ATR fine targeting (i.e. after the sighting device has been oriented to the destination such that the center of the ATR measurement radiation reflected spot coincides with the setpoint position—corresponding to the target axis—on the ATR area sensor), the sighting device can then continue to track movements by the target “live” and at appropriate speed such that the center of the ATR measurement radiation reflected spot continues to be as accurate as possible and always at the setpoint position on the ATR area sensor. The target is then often referred to as being “locked”. Problems may arise in this case when the target moves with such jerkiness and speed that it disappears from the visual range of the ATR detector (i.e. no further ATR measurement radiation reflected at the destination impinges on the ATR area sensor).
By way of example, EP 2 141 450 describes a surveying appliance having a function for automatically targeting a retroreflecting target and having an automatic target tracking functionality. In order to keep the target in the “locked” state and in order not to lose it from the visual range of the fine targeting detector, even in the case of rapid and jerky movements, it is proposed in this case that images of the target be taken in parallel by a camera (that is sensitive in the visible wavelength range) and that image processing be used to track movements by the target (or movements by objects moving together with the target), and thereby that recovery and relocking of the retroreflector be facilitated in the event of the target being lost from the “locked” state.
As an alternative to surveying retroreflecting targets using ATR fine targeting functions as described, the surveying of retroreflectorless targets is also known. However, since targets of such type—when impacted by an ATR measurement beam (as described above)—would reflect this beam diffusely and hence this would mean that a reflected spot that could be evaluated sufficiently in terms of an impingement position is not produced on the ATR area sensor by a long way, such targeting functions as are based on the principle of active impacting of the target with measurement radiation and detection and evaluation of an impingement position for the measurement radiation reflected at the target (such as those described above) cannot be used for retroreflectorless targets. A main disadvantage is therefore the requirement—which exists for surveying appliances from the prior art—of purely manual fine sighting of such retroreflectorless targets, which both often results in insufficient accuracy for the measurement of the spatial angles of the target (since the accuracy for the fine sighting of the target is then dependent on the skill and judgement by eye of a surveyor/user) and requires a high level of complexity (since sufficient patience, circumspection and care are indispensable for manual fine sighting of the target by the user). The manual fine targeting that is required for retroreflectorless targets is thus complex, time-consuming, unreliable and not very robust.
Furthermore, the prior art also discloses methods wherein an image and image processing are used (particularly using a target object position that has been determined in the image taken and on the basis of a known direction in which the image was taken, or a direction that is captured in the process) to ascertain the spatial angles of the measuring appliance with respect to the target object. Such methods are described in WO 2005/026767 or in WO 2006/053837, for example.
In summary: when using appropriate reflectors (particularly retroreflecting prisms) as target objects, automatic ATR fine targeting and target tracking using surveying appliances from the prior art thus works sufficiently well and reliably. The only great disadvantage in this regard is the indispensable need to use such retroreflecting target objects as can be manufactured only with a high level of complexity, such as 360-degree prisms, which are constructed from six single prisms, for example, and in such a manner ensure precise retroreflection of the ATR measurement beam. Accordingly, the costs of manufacture for such retroreflecting 360-degree prisms of very complex design are high. In addition, target objects made from glass are comparatively susceptible to destruction. By contrast, the geodetic surveying of retroreflectorless targets requires manual fine sighting of the target, which is therefore not very robust, not very reliable, not very precise and complex to perform.